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Three-Fold Intramolecular Ring-Closing Metatheses Involving Square-Planar Platinum Complexes with cis-Phosphorus Donor Ligands: Syntheses, Structures, and Properties of Parachute-like Complexes Katrin Skopek,† Michał Barbasiewicz,† Frank Hampel,† and John A. Gladysz*,‡ Institut für Organische Chemie and Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-UniVersität Erlangen-Nürnberg, Henkestrasse 42, 91054 Erlangen, Germany, and Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012, College Station, Texas 77842-3012 Received December 12, 2007
Reactions of the phosphite and phosphine complexes cisPtCl2((PX(CH2)mCHdCH2)3)2 (X/m ) O/3, O/4, O/5, -/5, -/6) with Grubbs’ catalyst, followed by hydrogenations, yield cis-PtCl2(P(X(CH2)2m+2X)3P) (6–40%). Crystal structures establish parachutelike motifs in which one X(CH2)2m+2X bridge lies roughly in the platinum coordination plane, and the others lie above and below.
Over the past decade, there has been an explosion of interest in alkene metatheses in metal coordination spheres.1 These efforts are often directed at what might be termed “designer” inorganic or organometallic molecules with specific target properties.2–11 Sauvage and co-workers pioneered this field with many elegant syntheses of catenanes and related topologically novel molecules.2 We have focused * To whom correspondence should be addressed. E-mail: gladysz@ mail.chem.tamu.edu. † Universität Erlangen-Nürnberg. ‡ Texas A&M University. (1) Bauer, E. B.; Gladysz, J. A. In Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 2, pp 403–431. (2) (a) Chambron, J.-C.; Collin, J.-P.; Heitz, V.; Jouvenot, D.; Kern, J.M.; Mobian, P.; Pomeranc, D.; Sauvage, J.-P. Eur. J. Org. Chem. 2004, 1627. (review of work from this group). (b) Frey, J.; Kraus, T.; Heitz, V.; Sauvage, J.-P. Chem. Commun. 2005, 5310. (3) Recent lead references: (a) Martinez, V.; Blais, J.-C.; Bravic, G.; Astruc, D. Organometallics 2004, 23, 861. (b) Chase, P. A.; Lutz, M.; Spek, A. L.; van Klink, G. P. M.; van Koten, G. J. Mol. Catal. A 2006, 254, 2. (c) Song, K. H.; Kang, S. O.; Ko, J. Chem.sEur. J. 2007, 13, 5129. (d) Zheng, X.; Jones, C. W.; Weck, M. J. Am. Chem. Soc. 2007, 129, 1105. (e) Chen, W.-Z.; Protasiewicz, J. D.; Davis, S. A.; Updegraff, J. B.; Ma, L.-Q.; Fanwick, P. E.; Ren, T. Inorg. Chem. 2007, 46, 3775. (f) Sivaramakrishna, A.; Su, H.; Moss, J. R. Angew. Chem., Int. Ed. 2007, 46, 3541. (4) (a) de Quadras, L.; Bauer, E. B.; Mohr, W.; Bohling, J. C.; Peters, T. B.; Martín-Alvarez, J. M.; Hampel, F.; Gladysz, J. A. J. Am. Chem. Soc. 2007, 129, 8296. (b) de Quadras, L.; Bauer, E. B.; Stahl, J.; Zhuravlev, F.; Hampel, F.; Gladysz, J. A. New J. Chem. 2007, 31, 1594. (5) (a) Bauer, E. B.; Hampel, F.; Gladysz, J. A. Organometallics 2003, 22, 5567. (b) Shima, T.; Bauer, E. B.; Hampel, F.; Gladysz, J. A. Dalton Trans. 2004, 1012. (c) Lewanzik, N.; Oeser, T.; Blümel, J.; Gladysz, J. A. J. Mol. Catal. A 2006, 254, 20. (6) Shima, T.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2004, 43, 5537. (7) (a) Nawara, A. J.; Shima, T.; Hampel, F.; Gladysz, J. A. J. Am. Chem. Soc. 2006, 128, 4962. (b) Nawara-Hultzsch, A. J. Doctoral Dissertation, Universität Erlangen-Nürnberg, Erlangen, Germany, 2008.
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much attention on assemblies that resemble insulated molecular wires4 and gyroscopes.6–11 The latter have been prepared by three-fold intramolecular ring-closing metatheses of trans adducts of phosphorus donor ligands, or I f II in Scheme 1. Such macrocyclizations are particularly efficient for trigonal-bipyramidal Fe(CO)3 adducts (III)6,11 but also can be applied to square-planar7–9 and octahedral10 targets (IV and V), provided that the ancillary ligands are small. During the course of these efforts, it became of interest to gauge the conformational flexibility of the diphosphorus ligands in II-V, which in certain cases can be detached from the metal,7 and more deeply probe certain dynamic properties. For example, rotation about the P-M-P axis in II can obviously exchange the ancillary ligands Ly. However, might geometric (e.g., trans/cis) isomerism also provide an exchange mechanism? Can the diphosphorus ligands otherwise contort, for example, in motifs that would allow both lone pairs to simultaneously bind to a surface? With the hope of probing such issues, we set out to examine alkene metatheses of related complexes in which the phosphorus donor ligands were cis, such as VI f VII in Scheme 1. As shown in Scheme 2, reactions of PtCl2 and the alkenecontaining trialkylphosphites P(O(CH2)mCHdCH2)3 (1; m ) 3 (a), 4 (b), 5 (c))11 afforded the adducts cis-PtCl2(P(O(CH2)mCHdCH2)3)2 (cis-2a-2c) in 60–95% yields as light-yellow or colorless oils after chromatographic workups. Complexes cis-2a-2c, and all new compounds below, were characterized by NMR (1H, 13C, and 31P) and mass spectrometry, as summarized in the Supporting Information. The 1J PPt values (5696–5698 Hz) implied cis isomers, in accordance with other syntheses of dihaloplatinum bis(phosphite) complexes.12–15 This likely reflects the cis/trans thermodynamic stabilities,12a and possible contributing electronic factors have been analyzed.12b (8) Wang, L.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2006, 45, 4372. (9) Wang, L.; Shima, T.; Hampel, F.; Gladysz, J. A. Chem. Commun. 2006, 4075. (10) Hess, G.; Hampel, F.; Gladysz, J. A. Organometallics 2007, 26, 5129. (11) Skopek, K.; Gladysz, J. A. J. Organomet. Chem. 2008, 693, 857.
10.1021/ic702391m CCC: $40.75
2008 American Chemical Society Published on Web 04/02/2008
COMMUNICATION Scheme 1. Syntheses of Gyroscope-like Complexes in Which trans-Phosphorus Atoms are linked by Three Bridges (Top, Middle) and Possible Extensions to Parachute-like Cis Analogues (Bottom)
Figure 1. Thermal ellipsoid diagrams (50% probability level) for cis-3a (top; one of two independent molecules in the unit cell) and cis-3b (bottom).
Scheme 2. Syntheses of Phosphite Complexes
Next, CH2Cl2 solutions of cis-2a-2c (0.79–0.99 mM) and Grubbs’ catalyst (10–20 mol %) were refluxed. After 12–36 h, the metathesis products were chromatographed and treated with H2 (5 bar) and Wilkinson’s catalyst (15–20 mol %). As shown in Scheme 2, workups gave the target complexes cis-PtCl2(P(O(CH2)nO)3P) (cis-3a-3c; n ) 2m + 2) in 10–20% overall yields. These feature 13-membered (cis-3a) to 17-membered (cis-3c) macrocycles in which the phosphorus donor atoms are tethered by three bridges of 10-14 atoms. Certain NMR properties were inconsistent with trans
isomers. For example, two sets of OCH2 13C NMR signals (2:1) and up to three sets of OCH2 1H NMR signals (1:1:1 or 2:1) were observed. With gyroscope-like IV, X-M-X′ rotation is rapid on the NMR time scale (room temperature to -80 °C), resulting in a single set of signals.7 To verify these assignments, the crystal structures of cis3a and -3b were determined (Supporting Information). The former featured two independent molecules in the unit cell and the latter a C2 symmetry axis. Thermal ellipsoid diagrams are depicted in Figure 1. In each complex, there is a “middle” bridge connecting the two phosphorus atoms that lie nearest to the platinum coordination plane. The outer bridges lie above and below the coordination plane, partially shielding the axial sites. As illustrated by VIII (Scheme 1), this generates a parachute-like motif, in which the P2PtCl2 moiety resembles the arms, body, and legs of the parachutist. The bond lengths and angles about the platinum, phosphorus, and (12) (a) Ahmad, N.; Ainscough, E. W.; James, T. A.; Robinson, S. D. J. Chem. Soc., Dalton Trans. 1973, 1148. Only when the PtX2(P(OAr)3)2 complexes in this paper bear o-aryl substituents can equilibrium concentrations of trans isomers be detected. (b) Crispini, A.; Harrison, K. N.; Orpen, A. G.; Pringle, P. G.; Wheatcroft, J. R. J. Chem. Soc., Dalton Trans. 1996, 1069. (13) Crystal structure of cis-PtCl2(P(OMe)3)2: Bao, Q.-B.; Geib, S. J.; Rheingold, A. L.; Brill, T. B. Inorg. Chem. 1987, 26, 3453. (14) For crystal structures of cis-PtCl2(P(OAr)3)2 species, see ref 12b and Sabounchei, S. J.; Naghipour, A. Molecules 2001, 6, 777. (15) (a) It has been shown that cis-dihaloplatinum bis(phosphine) complexes exhibit greater 1JPPt values than trans isomers: Grim, S. O.; Keiter, R. L.; McFarlane, W. Inorg. Chem. 1967, 6, 1133. (b) The 1JPPt values of two trans-dihaloplatinum bis(phosphite) complexes have been reported (4680–4405 Hz)12a,15c and are less than those of cis-2a-2c and -3a-3c. (c) van der Vlugt, J. I.; Ackerstaff, J.; Dijkstra, T. W.; Mills, A. M.; Kooijman, H.; Spek, A. L.; Meetsma, A.; Abbenhuis, H. C. L.; Vogt, D. AdV. Synth. Catal. 2004, 346, 399.
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COMMUNICATION Scheme 3. Syntheses of Phosphine Complexes
Figure 2. Selected limiting structures for complexes of dibridgehead phosphites and phosphines.
oxygen atoms are similar to those of other dichloroplatinum bis(phosphite) complexes (Supporting Information).13,14 We next turned our attention to metatheses of analogous cis-phosphine complexes. These have the potential of affording isomers of previously reported gyroscope-like complexes trans-PtCl2(P((CH2)n)3P) (trans-5).7 As shown in Scheme 3 (left), K2PtCl4 and P((CH2)mCHdCH2)3 (m ) 5 (c), 6 (d)6 were combined in the polar solvent water. Chromatographic workups produced two bands, the first providing the known isomers trans-PtCl2(P(CH2)mCHd CH2)3)2 (trans-4c and -4d)7 and the second the new species cis-4c and -4d in 70% and 33% yields, respectively. The 1 JPPt values confirmed the stereochemistry (3511–3510 Hz vs 2381 Hz for trans-4d).15a For reference, the synthesis of trans-4d, which is conducted in a nonpolar solvent, is also depicted in Scheme 3. Complexes cis-4c and -4d were subjected to metathesis/ hydrogenation sequences analogous to those in Scheme 2. Products arising from three-fold intramolecular metatheses, cis-5c and -5d, could be isolated in 6% and 40% yields, respectively. Complex cis-5c features 15-membered macrocycles, as opposed to the 17-membered macrocycles in the phosphite complex with the same number of methylene groups, cis-4c. Complex cis-5d features 17-membered macrocycles. As shown in Scheme 3 (right), when an analogous sequence is conducted with trans-4d, trans-5d is obtained in a yield comparable to that of cis-5d.7 The structures of cis-5c and -5d were supported by the 1 JPPt values (3568–3543 Hz vs 2398 Hz for trans-5d)15a and the observation of two PCH2 13C NMR signals (2:1). The mass spectra showed intense molecular ions and the successive loss of chloride ligands; unlike the precursors cis-4c and -4d, no ions corresponding to free phosphines were detected. However, samples suitable for crystallography have not yet been obtained. Nonetheless, cis-5c and -5d and cis-
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3a-3c establish that the macrocyclic dibridgehead diphosphines or diphosphites are extremely flexible. As shown in Figure 2, they readily adopt conformations XI in which the lone-pair vectors are twisted by 270° with respect to the reference structure IX. Hence, adducts of ligands with less twisted conformations, such as X, should also be possible. Importantly, the availability of both cis-5d and trans-5d allows the energy barriers and thermodynamics associated with cis/trans isomerization to be probed. Thermogravimetric analysis measurements with cis-5d showed an onset of mass loss at 204 °C, and differential scanning calorimetry experiments indicated a melting endotherm at 200 °C. Analogous measurements with trans-5d showed an onset of mass loss at 284 °C, a melting endotherm at 115 °C, and no other transitions prior to mass loss.7 These data do not suggest any isomerization, which would be evidenced by an exotherm with the less stable species. As a check, thermolyses were conducted. In one series of experiments, NMR tubes were charged with 1,2-dichlorobenzene solutions of cis-5d and trans-5d and kept at 180 °C for 14 h. In a second, solid samples were similarly heated, and then CDCl3 was added. The samples were analyzed by 31P NMR. The experiments with cis-5d showed 10% conversion to a second species with a signal coincident with that of trans-5d. Those with trans5d showed either no conversion (solid state) or 8% conversion (1,2-dichlorobenzene) to an unidentified species. Hence, there is no low-energy cis/trans isomerization pathway, excluding the involvement of such equilibria in ligand exchange or other dynamic processes. In summary, platinum complexes that contain cis-phosphite or -phosphine ligands with suitable alkene substituents undergo three-fold intramolecular alkene metatheses to give architecturally unusual adducts with parachute-like geometries. Some of these constitute isomers of previously reported gyroscope-like complexes. The ligands partially screen the axial sites associated with the platinum square plane, an attribute that can enhance the stabilities of certain types of derivatives,16 and might lead in other d8 metal complexes to catalysts with unusual selectivities or activities. Efforts in these directions, and extensions to other types of cis coordination geometries, are in progress and will be reported in due course. Acknowledgment. We thank the Deutsche Forschungsgemeinschaft (DFG; Grant GL 300/9-1) and the Humboldt Foundation (fellowship to M.B.) for support. Supporting Information Available: Experimental procedures, complex characterization, and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org. IC702391M (16) Klein, A.; Kaim, W. Organometallics 1995, 14, 1176.
